Meta-iodobenzylguanidine (MIBG), has a combination of the structural features of bretyllium and guanethedine. MIBG has been found to be a potential adrenomedullary agent as well as a radiopharmaceutical for other diagnostic and therapeutic applications. For example, MIBG, radiolabeled with iodine-131 and iodine-123, has been used in the diagnosis of heart abnormalities and neuroendocrine tumors. [131I]MIBG also has been used for the treatment of neuroendocrine tumors. With the currently used method (isotopic exchange), radioiodinated MIBG cannot be obtained at a no-carrier-added (n.c.a.) level. This may diminish its clinical utility for some applications. there are some shortcomings in [131I]MIBG therapy, especially for neuroblastoma, attributed to the physical characteristics of iodine-131. The physical characteristics of astatine-211 could be more suitable for the treatment of neuroblastoma since astatine-211 emits alpha particles of high linear energy transfer. It may be advantageous to have MIBG analogs labeled with positron- emitting nuclides for positron emission tomographic imaging. The objectives of this proposal are to develop synthetic methods for the preparation of n.c.a. [131I]MIBG,3- [211At]astatobenzylguanidine([211At]MABG)3- [18F]fluorobenzylguanidine([18F]MFBG) and 4-[18F]fluoro-3- iodobenzylguanidine([18F]FIBG) and to evaluate their potential as diagnostic and therapeutic radiopharmaceuticals. First, the in vitro binding characteristics of each of the new agents will be studied using neuroblastoma cell lines. Biodistribution of these agents will be determined initially in normal mice and subsequently in athymic mouse human neuroblastoma xenograft models. These studies should give information on the potential utility of these agents as MIBG analogs.